Carrier tracking without pilots

ABSTRACT

A carrier tracking technique includes allocating a first number of bits per symbol to a carrier tracking subcarrier of a plurality of subcarriers of an orthogonal frequency division multiplexing (OFDM) signal based on a first target performance margin. The technique includes allocating numbers of bits per symbol to other subcarriers of the plurality of subcarriers based on a second target performance margin.

BACKGROUND

1. Field of the Invention

The present invention relates to communications systems, and moreparticularly to carrier tracking in communications systems.

2. Description of the Related Art

In a typical orthogonal frequency division multiplexing (OFDM)communications system, a carrier frequency oscillator used to modulate abaseband signal in a transmitter of the communication system isindependent from a carrier frequency oscillator used by a receiver todemodulate the baseband information from a received modulated signal. Inaddition, channel effects introduce frequency offset (i.e., a differencein frequency of a transmitted subcarrier and a received subcarrier) intoa received OFDM signal. A frequency offset between a received subcarrierand a frequency of a transmitted subcarrier may result in amplitudereduction of a received symbol and loss of orthogonality betweensubcarriers of the received OFDM signal, which results in inter-carrierinterference. To avoid severe system performance degradation, anyuncompensated frequency offset must not exceed a small fraction of thesubcarrier signaling rate. However, small carrier offsets (i.e., on theorder of a percentage of the subcarrier spacing) can severely degradeOFDM demodulation.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In at least one embodiment of the invention, a method includesallocating a first number of bits per symbol to a carrier trackingsubcarrier of a plurality of subcarriers of an orthogonal frequencydivision multiplexing (OFDM) signal based on a first target performancemargin. The method includes allocating numbers of bits per symbol toother subcarriers of the plurality of subcarriers based on a secondtarget performance margin.

In at least one embodiment of the invention, an apparatus includes aselect module configured to select a carrier tracking subcarrier from aplurality of subcarriers of an orthogonal frequency divisionmultiplexing (OFDM) signal based on performance estimates for theplurality of subcarriers. The apparatus includes a bit allocation moduleconfigured to allocate a number of bits per symbol to the carriertracking subcarrier based on a first target performance margin. The bitallocation module is configured to allocate numbers of bits per symbolto other subcarriers of the plurality of subcarriers based on a secondtarget performance margin.

In at least one embodiment of the invention, a method includescommunicating an orthogonal frequency division multiplexing (OFDM)signal over a channel. A carrier tracking subcarrier of a plurality ofsubcarriers of the OFDM signal is modulated by a first number of databits per symbol. Other subcarriers of the plurality of subcarriers aremodulated by numbers of data bits per symbol. The first number of databits per symbol is based on a first target performance margin. Thenumbers of data bits per symbol are based on a second target performancemargin.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a functional block diagram of an exemplarycommunications system.

FIG. 2 illustrates an exemplary logical topology of node devices of thecommunications system of FIG. 1.

FIG. 3 illustrates a functional block diagram of an exemplary nodedevice of the communications system of FIG. 1.

FIG. 4 illustrates a functional block diagram of an exemplarytransmitter path of the node device of FIG. 3, consistent with at leastone embodiment of the invention.

FIG. 5 illustrates a functional block diagram of an exemplary receiverpath of the node device of FIG. 3, consistent with at least oneembodiment of the invention.

FIGS. 6A, 6B, and 6C illustrate information and control flows forselecting carrier tracking subcarriers for use in a carrier trackingtechnique consistent with at least one embodiment of the invention.

FIG. 7 illustrates an exemplary signal-to-noise ratio estimate andexemplary bit allocations consistent with at least one embodiment of theinvention.

FIG. 8 illustrates information and control flows for generating afrequency offset and timing adjustment based on data communicated overcarrier tracking subcarriers for a carrier tracking technique consistentwith at least one embodiment of the invention.

FIG. 9 illustrates a functional block diagram of an exemplary carriertracking module consistent with at least one embodiment of theinvention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Referring to FIG. 1, in an exemplary digital communications network(e.g., network 100) nodes (e.g., nodes 108, 110, 112, and 114) andsplitters (e.g., splitters 104 and 106) are configured as a local areanetwork (e.g., network 101) using communications over a channel (e.g.,coaxial cables). In at least one embodiment of network 100, nodes 108,110, 112, and 114 communicate with a wide-area network (e.g., cableprovider 102) via splitter 106 and/or splitter 104. In addition, in atleast one embodiment of network 100, nodes 108, 110, 112, and 114communicate with each other via splitter jumping.

Note that due to effects of splitter jumping and reflections atdifferent terminations of network 101, channel characteristics (e.g.,attenuation and delay) for a link between two nodes may be differentfrom the channel characteristics for a link between two other nodes. Inaddition, channel characteristics in a forward path may be differentfrom channel characteristics in a reverse path. Thus, channel capacitybetween each source node and destination node varies from the channelcapacity for two other source nodes and destination nodes. Accordingly,to appropriately use channel capacity of network 101, individual nodesof network 101 determine and store suitable separate physical (PHY)parameters tailored for each link (i.e., store separate PHY profiles foreach link). Referring to FIG. 2, a logical model of network 101 is afully-meshed collection of point-to-point links Each link has uniquechannel characteristics and capacity. In addition to point-to-pointcommunications, network 101 supports broadcast and multicastcommunications in which a source node uses a common set of PHYparameters that may be received by all destination nodes.

In at least one embodiment of network 101, nodes 108, 110, 112, and 114share a physical channel. Thus, only one node is allowed to transmit ata particular time. For example, the physical channel is timedivision-duplexed and coordinated by a Media Access Control (MAC) datacommunication protocol sublayer using time division multiple access(TDMA). In at least one embodiment, network 101 is a centrallycoordinated system with one node being a network-coordinator (NC). Anode that is the NC transacts data on the network like any other node,but is also responsible for transmitting beacons to advertise networkpresence and timing, coordinating a process for admitting nodes to thenetwork, scheduling and coordinating transmission of data among allnodes in the network, scheduling and coordinating link-maintenanceoperations (e.g., operations during which nodes update their physicalprofiles), and other functions.

Referring to FIG. 3, an exemplary node 102 includes a processorconfigured to generate and process data communicated over network 101.Data to be transmitted over the network is digitally processed intransmitter 306 and transmitted over the channel using RF transmitter310. In at least one embodiment, node 102 includes a radio frequencyreceiver configured to receive analog signals over the channel and toprovide a baseband analog signal to the receiver path (e.g., receiver308), which digitally processes the baseband signal to recover data andcontrol information symbols and provide it to processor 304.

In at least one embodiment, network 101 implements orthogonal frequencydivision multiplexing (OFDM). In general, OFDM is a frequency-divisionmultiplexing scheme utilized as a digital multi-carrier modulationmethod in which a large number of orthogonal subcarriers havingclosely-spaced frequencies are used to carry data. The data is dividedinto several parallel data streams or channels (i.e., frequency bins orbins), one for each sub-carrier. Each subcarrier is modulated with aconventional modulation scheme (e.g., quadrature amplitude modulation orphase shift keying) at a low symbol rate, maintaining total data ratessimilar to conventional single-carrier modulation schemes in the samebandwidth. In at least one embodiment of node 102, the physicalinterface (e.g., transmitter 306 and receiver 308) utilizes adaptiveconstellation multi-tone (ACMT), i.e., node 102 pre-equalizes modulationto the frequency response of each link using bit loaded orthogonalfrequency division multiplexing (OFDM). In addition, channel profilingtechniques tailor the modulation for each link. In at least oneembodiment of node 102, physical layer channels are approximately 50 MHzwide (i.e., the ACMT sampling rate is approximately 50 MHz) and thetotal number of OFDM subcarriers is 256. However, other sampling ratesand numbers of subcarriers may be used. In at least one embodiment ofnode 102, due to DC and channel edge considerations, only 224 of the 256subcarriers are available for typical communications.

In at least one embodiment of node 102, a modulation profile isgenerated based on probe packets sent between nodes and analyzed at thereceiving nodes. After analysis, a receiving node assigns bits tosubcarriers for a particular link and communicates this information tonode 102. An individual ACMT subcarrier may be active or inactive (i.e.,turned off). An active ACMT subcarrier is configured to carry one toeight bit Quadrature Amplitude Modulation (QAM) symbols. In at least oneembodiment of node 102, transmit power of a sending node is dynamicallyadjusted based on modulation profiling using the probe packets and basedon link performance.

In general, the channel is time-varying and link maintenance operations(LMOs) facilitate the recalculation of PHY parameters. Thus, at regularintervals a transmitting node sends one or more probe packets which arereceived and analyzed by receiving nodes. The receiving nodes send backprobe reports to the corresponding transmitting nodes. Those probereports may include updated parameters. In at least one embodiment ofnode 102, each probe packet includes a preamble and a payload. In atleast one embodiment of node 102, multiple probe types are used forcharacterization of different network elements. In at least oneembodiment of node 102, probe and/or payload packets include a preamblethat includes one or more symbols used for channel estimation.

Referring to FIG. 4, in at least one embodiment, transmitter 306receives a frame of data from a Medium Access Control data communicationprotocol sub-layer (i.e., MAC layer). In at least one embodiment oftransmitter 306, a channel coding module (e.g., FEC padding module 402)encodes the MAC frame with redundancies using a predetermined algorithmto reduce the number of errors that may occur in the message and/orallow correction of any errors without retransmission. In at least oneembodiment of transmitter 306, an encryption module (e.g., encryptionmodule 404) encrypts the frame to deter eavesdropping and provide linklayer privacy. In at least one embodiment, encryption module 404implements 56-bit Data Encryption Standard (DES) encryption using aprivacy key generated and received from the NC. However, in otherembodiments of transmitter 306, other encryption techniques may be used.

In at least one embodiment of transmitter 306, an encoder (e.g., forwarderror correction (FEC) encoder 406) encodes the frame into twoReed-Solomon codewords including a regular codeword and a shortenedcodeword to reduce the FEC padding. Note that in other embodiments oftransmitter 306, other types of forward error correction are used (e.g.,other block codes or convolutional codes). In at least one embodiment oftransmitter 306, a padding module (e.g., ACMT symbol padding module 408)inserts additional bits into the data to form symbols having aparticular ACMT symbol size. In at least one embodiment of transmitter306, a scrambler module (e.g., byte scrambler 410) scrambles the dataacross multiple bytes to change the properties of the transmitted datastream. For example, byte scrambler 410 facilitates data recovery byreducing dependence of the signal power spectrum on the actualtransmitted data and/or reducing or eliminating occurrences of longsequences of ‘0’ or ‘1’ that may otherwise cause saturation of digitalcircuitry and corrupt data recovery. In at least one embodiment oftransmitter 306, an ACMT subcarrier mapping module (e.g., subcarriermapper 424) maps bits of data to ACMT subcarriers according to apredetermined bit loading profile (e.g., a bit loading profile receivedfrom a receiving node and stored in memory). In at least one embodimentof transmitter 306, the predetermined profile is selected from aplurality of predetermined profiles according to a particular mode orpacket type (e.g., beacon mode, diversity mode, Media Access Plan (MAP),unicast, or broadcast) and link for transmission (e.g., a profile storedfor a particular receiving node).

In at least one embodiment of transmitter 306, a scrambler module (e.g.,bin scrambler 422) scrambles the data of the ACMT subcarriers to changethe properties of the transmitted data stream (e.g., reduce dependenceof the signal power spectrum on the actual transmitted data or to reduceor eliminate occurrences of long sequences of ‘0’ or ‘1’) to propertiesthat facilitate data recovery. A modulator (e.g., ACMT modulator 420)generates the time domain in-phase and quadrature (i.e., I and Q)components corresponding to the OFDM signal. ACMT modulator 420 includesan N-point IFFT and inserts a cyclic prefix to the modulated data (i.e.,inserts the cyclic prefix to time domain symbols). For example, ACMTmodulator 420 copies the last N_(CP) samples of the IFFT output (e.g., Nsamples) and prepends those samples to the IFFT output to form an OFDMsymbol output (e.g., N+N_(CP) samples). The cyclic prefix is used as aguard interval to reduce or eliminate intersymbol interference from aprevious symbol and also to facilitate linear convolution of the channelto be modeled as a circular convolution, which may be transformed to thefrequency domain using a discrete Fourier transform. This approachallows for simple frequency-domain processing, such as for channelestimation and equalization. The length of the cyclic prefix is chosento be at least equal to the length of the multipath channel. In at leastone embodiment of transmitter 408, filter 418 limits the frequency bandof the signal to a signal having a particular spectral mask prior todigital-to-analog conversion (e.g., by digital-to-analog converter 416)and any frequency modulation to a higher frequency band (e.g., frombaseband to one of four frequency bands in the range of 850 MHz to 1525MHz at 25 MHz increments) for transmission.

Depending upon a particular communication type, in at least oneembodiment of transmitter 306, frequency domain preamble generator 414or time domain preamble generator 412 inserts a preamble into the packetprior to processing a MAC frame of data. For example, rather thanprocessing a MAC frame through the portion of the transmitter pathincluding FEC padding module 402, encryption module 404, FEC encoder406, ACMT symbol padding module 408, and byte scrambler 410, analternate source (e.g., frequency domain preamble generator 414)provides a plurality of frequency domain preamble symbols, including oneor more frequency domain symbols (e.g., which are generated or retrievedfrom a storage device) to subcarrier mapper 424. Subcarrier mapper 424maps bits of those frequency domain preamble symbols to individualsubcarriers. Those frequency domain preamble symbols are then processedby the remainder of transmitter 306 (e.g., bin scrambled, ACMTmodulated, filtered, and converted to an analog signal) and sent to RFTX 310 for transmission. The frequency domain preamble symbols provide areference signal that may be used by the receiver for timing andfrequency offset acquisition, receiver parameter calibration, and PHYpayload decode. In at least one embodiment of transmitter 306, frequencydomain preamble generator 414 provides a plurality of channel estimationfrequency domain symbols (e.g., two channel estimation symbols) tosubcarrier mapper 424, bin scrambler 422, ACMT modulator 420, filter418, and DAC 416. In at least one embodiment of transmitter 306, timedomain preamble generator 412 inserts a plurality of time domain symbolsdirectly to filter 418 for digital-to-analog conversion and thentransmission over the link. The time domain preamble symbols provide areference signal that may be used by the receiver to identify packettype and for coarse timing and frequency offset acquisition.

Referring to FIG. 5 in at least one embodiment, receiver 308 receives ananalog signal from the RF receiver interface (e.g., RF receiver 312 ofFIG. 3) and analog-to-digital converter (e.g., ADC 502) convertsin-phase and quadrature analog signal components of the received signalinto a complex digital signal. Referring back to FIG. 5, in at least oneembodiment of receiver 308, one or more filters (e.g., RX filters 504)limit the complex digital signal to a baseband signal having aparticular bandwidth. In at least one embodiment of receiver 308, one ormore other filters (e.g., high-pass filter 506) attenuates or removes aDC component of the complex digital signal. In at least one embodimentof receiver 308, I/Q balance module 508 adjusts the real and imaginarycomponents of the complex baseband signal, which are balanced at thetransmitting node, but become unbalanced after transmission over thechannel by analog circuitry. I/Q balance module 508 adjusts the in-phaseand quadrature components of the complex baseband signal to haveapproximately the same gain. In at least one embodiment of receiver 308,a timing module (e.g., timing interpolator module 510) adjusts thesample timing based on a timing offset, e.g., by using a delay filter tointerpolate samples and generate an output sample having a particulartiming based on the timing offset. In at least one embodiment ofreceiver 308, a frequency offset correction module (e.g., de-rotatemodule 512) compensates for any frequency offset, e.g., by performing acomplex multiply of the received data with a complex data value based ona previously determined target angle of rotation that compensates forthe frequency offset. In at least one embodiment of receiver 308, amodule (e.g., cyclic prefix removal module 514) strips a number ofsamples (e.g., N_(CP) samples, where N_(CP) is the number of samplesinserted by the transmitter as the cyclic prefix) from the de-rotateddata symbol and provides the resulting time domain symbol to ademodulator (e.g., fast Fourier transform (FFT) module 516), whichgenerates frequency domain symbols.

In at least one embodiment of receiver 308, during data demodulation anddecode sequences, a frequency domain equalizer (e.g., FEQ 546) reduceseffects of a bandlimited channel using frequency domain equalizer tapsgenerated by a channel estimation module (e.g., channel estimator 548),as described further below. In at least one embodiment of receiver 308,during data demodulation and decode sequences that communicate in adiversity mode (e.g., a mode in which the same signal is transmitted bymultiple subcarriers) diversity combiner module 544 combines signalsrepeated on multiple subcarriers into a single improved signal (e.g.,using a maximum ratio combining technique). In at least one embodimentof receiver 308, frequency domain symbols are demapped from thesubcarriers and descrambled (e.g., using demapper/descrambler module542) according to a technique consistent with the mapping and scramblingtechnique used on a transmitting node. The demapped and descrambled bitsare decoded (e.g., using decoder 540) consistent with coding used by atransmitting node. A decryption module (decryptor 538) recoversdemodulated bits and provides them to a processor for furtherprocessing.

In at least one embodiment of receiver 308, during timing and frequencyacquisition sequences, a gain control module (e.g., automatic gaincontrol module 522) provides power adjustment signals to monotonicallyadjust analog gain of the RF receiver interface (e.g., RF receiver 312of FIG. 3) using adjustments in a particular range and step size.Referring back to FIG. 5, in at least one embodiment of receiver 308,during timing and frequency acquisition sequences, the output ofde-rotator 512 is stored in a storage device (e.g., buffer 528). Thestored data is used to detect a preamble of a packet (e.g., usingpreamble detection module 526). In at least one embodiment, a frequencyoffset and timing acquisition module (e.g., frequency offset and timingacquisition module 530) generates an indication of a start of a symboland an indication of a frequency offset for use by the receiver forrecovery of subsequent received symbols (e.g., timing interpolator 510,de-rotator 512, and cyclic prefix removal module 514).

In at least one embodiment of receiver 308, during channel estimationsequences (e.g., during receipt of symbols of a probe signal) asignal-to-noise ratio (SNR) estimator (e.g., SNR estimator 518)generates an SNR estimate based on multiple frequency domain symbols. Abit loading module (e.g., bit loading module 520) assigns a number ofbits for transmission over individual subcarriers of the OFDM channelbased on the SNR estimate. For example, bit loading module 520 turns offan individual subcarrier or assigns a one to eight bit QAM symbol to theindividual subcarrier. In general, bit loading module 520 generates abit allocation for each subcarrier of an OFDM signal and receiver 308communicates those bit loading assignments to a transmitting node for aparticular link for generating packets for communication during datacommunications intervals. In addition, the resulting bit loading isstored in receiver 308 for data recovery during subsequentcommunications sequences.

In general, FEQ 546 reduces effects of a bandlimited channel byequalizing the channel response. In at least one embodiment of receiver308, a payload packet includes a preamble portion that includes one ormore symbols for channel estimation. A typical channel estimation symbolis generated using a pseudorandom number generator, obtained from astorage device, or generated using another suitable technique. In atleast one embodiment of receiver 308, channel estimator 548 estimatesthe channel response based on received channel estimation symbols.Channel estimator 548 determines frequency domain equalizer coefficientsbased on that estimated channel response (i.e., channel responseestimate) and provides the frequency domain equalizer coefficients toFEQ 546 for use during data demodulation and decode sequences.

In a typical OFDM system, a transmitter oscillator frequency andsubcarrier frequencies are related by integers. A technique forsynchronizing subcarriers at the receiver to subcarriers generated at atransmitting node uses at least one frequency offset determinationduring an acquisition interval (e.g., coarse and fine frequency offsetdeterminations). For example, a coarse frequency offset determinationtechnique resolves offsets greater than ½ of the subcarrier spacing, andfine frequency offset determinations resolves offsets up to ½ of thesubcarrier spacing. In at least one embodiment of receiver 308,frequency offset and timing acquisition module 530 performs both coarsefrequency acquisition and fine frequency acquisition to determine timingoffsets and frequency offsets using known sequences received as part ofa packet preamble. The coarse and fine frequency acquisition techniquesuse known symbols and require channel stationarity for the correspondingtime interval. However, once the preamble of a packet is over, slightchanges in the overall system may result in additional frequency offset.Accordingly, a carrier tracking module (e.g., carrier tracking module550) implements a decision-directed, frequency tracking technique todetermine frequency offsets for reliable data demodulation thatcompensates for the time-varying nature of carriers to achieve andmaintain a target system performance level (e.g., target bit-errorrate).

In an exemplary receiver 308, carrier tracking module 550 uses a pilotsubcarrier (n_(P)) that carries known training data to synchronize thefrequency and phase of the receiver clock with the transmitter clock. Atypical pilot tone transmitted from the source node has only a realcomponent (i.e., the imaginary component is zero) and the imaginary partof the complex output of subcarrier n_(P) from the FFT is input to afeedback loop on the receiver. That feedback loop is configured toadjust the receive clock signal to drive to zero the recovered imaginarypart of the pilot tone. The imaginary part of the complex output ofsubcarrier n_(P) from the FFT is input to a loop filter, which via a DACdelivers a digital control signal to de-rotator 512 and timinginterpolator 510. However, in other embodiments of receiver 308, theoutput of the loop filter is a control voltage that is provided to aVCXO that adjusts the frequency of the receive clock. Note thattransmitter 306 and receiver 308 are exemplary only and othertransmitters and receivers consistent with teachings herein exclude oneor more modules or include one or more additional modules in thetransmit and receive paths, respectively.

As described above, a typical technique for carrier tracking during datademodulation and decode intervals uses training symbols communicatedover pilot tones, which dedicate power for carrier tracking at theexpense of signal power (i.e., the use of pilot tones results in a lossin data rate). Instead of dedicating one or more subcarriers to beingpilot tones that carry known data, receiver 308 selects one or moresubcarriers to be carrier tracking subcarriers that carry payload data.Frequency offset analysis using carrier tracking subcarriers that carrypayload data increases the spectral efficiency of the carrier trackingtechniques compared to techniques that use pilot tones carrying knowntraining data. As referred to herein, a carrier tracking subcarrier is asubcarrier of an OFDM signal that is used to communicate data with areduced likelihood of errors in data recovery as compared to othersubcarriers of the OFDM signal. In at least one embodiment of receiver308, carrier tracking module 550 is configured to generate receiverparameters (e.g., frequency offset adjustment, timing adjustment, and/orFEQ tap adjustment) for receiver elements (e.g., de-rotator 512, timinginterpolator 510, and/or FEQ 546, respectively) to improve carriertracking and received carrier orthogonality based on payload datareceived using one or more carrier tracking subcarriers.

Referring to FIGS. 5, 6A, 6B, and 6C, in at least one embodiment,receiver 308 receives N packets of known data over a particular link(602) and a performance estimator (e.g., signal-to-noise ratio (SNR)estimator 518) generates performance (e.g., SNR) estimates for eachsubcarrier of the OFDM signal based on the N received packets receivedover that particular link (604). A carrier tracking subcarrier selector(e.g., carrier tracking subcarrier selector 519) uses that performanceestimate to identify a predetermined number of subcarriers (e.g., N_(P)subcarriers) to be designated as carrier tracking subcarriers for thelink (606). The predetermined number of subcarriers may be a fixednumber of subcarriers or may be determined based on a programmable valuereceived from a storage device, or by other suitable technique. In atleast one embodiment, carrier tracking subcarrier selector 519identifies the N_(P) subcarriers with the best performancecharacteristics (e.g., highest SNR estimate values) to be designated ascarrier tracking subcarriers and provides the designation to bit loadingmodule 520. Other embodiments of carrier tracking subcarrier selector519 select N_(P) subcarriers based on subcarrier SNR estimate values,proximity of the subcarrier to an edge of the frequency band of the OFDMsignal, and/or a predetermined amount of spacing between carriertracking subcarriers to provide at least some frequency diversity. Forexample, the N_(P) subcarriers are selected from those subcarriershaving the highest SNR values that are at least a predetermined numberof subcarriers from the edge of the frequency band and are spaced atleast k subcarriers from the next adjacent carrier tracking subcarrier,where k is an integer.

In at least one embodiment of receiver 308, bit loading module 520assigns a number of bits to each subcarrier (607). Bit loading module520 determines the throughput of a link by assigning a particular numberof bits to be transported by each individual subcarrier of the OFDMsignal. For example, bit loading module 520 attempts to maximize atransport capacity of a particular link. In at least one embodiment, bitloading module 520 approximates a water-filling algorithm. In at leastone embodiment, bit loading module 520 uses a gap approximation on asubcarrier-by-subcarrier basis in an iterative manner or uses othersuitable technique to allocate bits to subcarriers. In general, the bitloading technique determines a number of bits that may be supported by aparticular subcarrier based on a performance estimate for thatparticular subcarrier and a target performance margin, which isdetermined based on a target bit error rate (BER) and a target codingscheme. In at least one embodiment of bit loading module 520, asignal-to-noise ratio (SNR) estimate is used as the performance estimateand indicator of subcarrier quality. As the SNR estimate for aparticular subcarrier increases, the number of bits that can besupported by the particular subcarrier increases for the targetperformance margin.

In at least one embodiment of bit loading module 520, those subcarriersdesignated as carrier tracking subcarriers use a different performancemargin than other subcarriers. In at least one embodiment of bit loadingmodule 520, the target performance margin used for a carrier trackingsubcarrier is substantially greater than (i.e., at least 3 dB greater,e.g., 8 dB greater) the target performance margin for other subcarriers.Accordingly, by using the substantially greater performance margin todetermine the bit loading on carrier tracking subcarriers, the number ofbits allocated to a subcarrier that is designated as a carrier trackingsubcarrier is reduced from the number of bits allocated to thatsubcarrier had it not been designated as a carrier tracking subcarrier.For example, the number of bits allocated when a subcarrier isdesignated as a carrier tracking subcarrier is at least one bit lessthan the number of bits allocated to that subcarrier had it not beendesignated as a carrier tracking subcarrier. Increasing the performancemargin for the carrier tracking subcarrier and reducing the number ofbits allocated to that subcarrier increases the likelihood of correctdecisions when demapping symbols received using the carrier trackingsubcarrier. Thus, the increased performance margin reduces thelikelihood of errors during recovery of data received over the carriertracking subcarrier, thereby facilitating use of unknown data forcarrier tracking.

Referring to FIG. 6B, in at least one embodiment, bit loading module 520generates margin-adjusted performance indicators based on the SNRestimates (608). In at least one embodiment, bit loading module 520accesses entries of a predetermined table indicating a number of bits tobe allocated for corresponding margin-adjusted performance indicators.For those subcarriers designated as carrier tracking subcarriers, bitloading module 520 uses another predetermined table based on the targetcarrier tracking subcarrier performance margin to determine a number ofbits allocated to the carrier tracking subcarriers. The margin-adjustedperformance indicators for those subcarriers designated as carriertracking subcarriers are generated by subtracting a target carriertracking subcarrier margin value from the SNR estimate corresponding tothose designated subcarriers and another target margin value issubtracted from the SNR estimates corresponding to other subcarriers.Then, bit loading module 520 allocates a number of bits to eachsubcarrier based on those margin-adjusted performance indicators foreach subcarrier (610). Note that the order of operations of FIGS. 6A and6B are exemplary only and the bit loading of carrier trackingsubcarriers and other subcarriers may be performed using variations ofthe operations of FIGS. 6A and 6B.

For example, referring to FIG. 6C, in another embodiment, bit loadingmodule 520 performs a bit loading of all subcarriers using a targetperformance margin and performance indicators for all subcarriers (609).Then, bit loading module 520 adjusts the bit loading for the designatedcarrier tracking subcarriers based on corresponding performanceindicators and a target carrier tracking performance margin (611). Bitloading module 520 accesses entries of a carrier tracking subcarrier bitloading table indicating numbers of bits to be allocated forcorresponding margin-adjusted performance indicators for carriertracking subcarriers and accesses another bit loading table for othersubcarriers. The carrier tracking subcarrier bit loading table is basedon a target carrier tracking performance margin and the other table usedfor other subcarriers is based on a target performance margin that issubstantially less than the target carrier tracking performance margin.The carrier tracking subcarrier table indicates numbers of bits formargin-adjusted performance indicators that result in an increasedlikelihood that decisions made by demapping symbols received using thecarrier tracking subcarrier are correct as compared to the other numbersof bits provided by the other bit loading table and used for bit loadingof the other subcarriers.

Referring to FIG. 7, an exemplary performance estimate (e.g., SNRestimate 702) and corresponding bit loadings are illustrated for an OFDMsignal including 24 subcarriers consistent with the carrier trackingtechnique described herein. In at least one embodiment, carrier trackingsubcarrier selector 519 determines, based on SNR values 702, thatsubcarrier 11 has the greatest SNR estimate value (SNR_(MAX)) anddesignates subcarrier 11 as a carrier tracking subcarrier. Carriertracking subcarrier selector 519 communicates this designation to bitloading module 520, which bit loads the subcarriers. Subcarrier 11 isbit loaded using a target carrier tracking subcarrier margin value toobtain a greater performance margin than other subcarriers, which arebit loaded using a target margin value that is substantially less thanthe target carrier tracking subcarrier margin value. Rather than beingallocated 8 bits, which requires an SNR of at least SNR₈ and results ina performance margin of MARGIN₈ (where MARGIN₈=SNR_(MAX)−SNR₈) forsubcarrier 11, bit loading module 520 allocates subcarrier 11 with only6 bits of data, which only requires an SNR of at least SNR₆ and resultsin a performance margin of MARGIN₆ (where MARGIN₆=SNR_(MAX)−SNR₆) forsubcarrier 11. Note that margin values will vary according to systemparameters and other numbers of carrier tracking subcarriers may beused.

Referring back to FIG. 6A, the node transmits the resulting bit loadingto a corresponding transmitting node for use during subsequent datatransmission. The node also stores the resulting bit loading in receiver308 for use during subsequent data recovery operations (612). Duringsubsequent data communications, a carrier tracking module generatessystem adjustment parameters for the link based on data received usingthe carrier tracking subcarriers (614).

Referring to FIGS. 5, 8, and 9, in at least one embodiment of receiver308, carrier tracking module 550 receives one or more symbols (e.g., Psymbols, X_(p)[k], where 1≦p≦P, where p and P are integers) receivedover P corresponding carrier tracking subcarriers and processes thosesymbols to determine receiver parameters for the link. Carrier trackingmodule 550 receives X_(p)[n] from FEQ 546 (802). Demapper 902 demapseach symbol to generate X _(p)[k]. For example, for each received symbolvalue for a particular subcarrier, carrier tracking module 550determines a closest symbol value of the possible symbol values for aparticular subcarrier modulation scheme. Conjugator 904 generates X_(p)*[k], which is the complex conjugate of the demapped symbol X_(p)[k] (806). Multiplier 906 multiplies the complex conjugate of thedemapped symbol with the original symbol received from FEQ 546 (i.e.,X_(p)[k]× X _(p)*[k]=Y_(p)[k]), to generate a resultant vector (808). Ifthere is no frequency offset, then the corresponding complex value willbe completely real and have no imaginary component. Summer 908 then sumsweighted products for all carrier tracking subcarriers (Y₀[k]+Y₁[k]+ . .. +Y_(p)[k]) (810) and then phase generator 910 computes the phase ofthe sum to generate a single phasor indicative of a total frequencyoffset, Y_(φ)[k] (812). In at least one embodiment of carrier trackingmodule 550, rather than computing weighted phasors and summing theweighted phasors to compute the frequency offset, carrier trackingmodule 550 determines individual phasors for each designated carriertracking subcarrier and averages those phasors to determine thefrequency offset.

In at least one embodiment of carrier tracking module 550, differencemodule 912 determines a difference from a prior symbol in timeY_(Δ)[k]=Y_(φ)[k]−Y_(φ)[k−1] to determine the time varying change infrequency offset (814). Filter 914 (e.g., a low pass filter) providesthe resulting phase difference to timing interpolator 510 and de-rotator512. In at least one embodiment, filter 914 has programmable parameters,which may be programmed to achieve a particular bandwidth andsensitivity of the carrier tracking For example, in embodiments ofcarrier tracking module 550 configured for a wider bandwidth, thecarrier tracking is noisier, but tracks frequency offset changes faster.Similarly, in embodiments of carrier tracking module 550 configured fora narrower bandwidth, the carrier tracking results in improved frequencyoffset estimates, but tracks changes slower. The output of filter 914 isan indicator of a frequency offset. Note that in other embodiments ofreceiver 308, carrier tracking module 550 receives the OFDM frequencydomain symbols from other modules of the receiver path (e.g.,demappper/descrambler 542, decoder 540, decryptor 538, or other suitablemodule) and functions of carrier tracking module 550 are adjusted basedon the processing state of the OFDM frequency domain symbols.

In at least one embodiment of receiver 308, a sampling clock and acarrier signal are generated by the same source. Accordingly, in atleast one embodiment, carrier tracking module 550 provides indicators oftime-varying frequency offset to timing interpolator 510 and de-rotator512, which adjust a sampling frequency offset and carrier frequencyoffset, respectively, of receiver 308. In at least one embodiment,carrier tracking module 550 provides an indication of carrier frequencyoffset to FEQ 546. Accordingly, de-rotator 512 compensates for thetime-varying carrier frequency offset using a complex multiply of theOFDM signal. FEQ 546 adjusts the FEQ taps according to a common phaseerror of the carrier tracking subcarriers. For example, FEQ 546multiplies each FEQ tap by an average phase error (e.g., using a singlephasor multiply for each FEQ tap) to de-rotate by the common phase errorto have a net phase error of zero after the correction. Thus, carriertracking techniques described herein use carrier tracking subcarriersthat carry payload data to determine a frequency offset indicator, whichis used to compensate for frequency offsets during data communicationsover a particular link. The carrier tracking techniques described hereinincrease the spectral efficiency of the carrier tracking techniques ascompared to other techniques that use pilot tones carrying knowntraining data.

While circuits and physical structures have been generally presumed indescribing embodiments of the invention, it is well recognized that inmodern semiconductor design and fabrication, physical structures andcircuits may be embodied in computer-readable descriptive form suitablefor use in subsequent design, simulation, test or fabrication stages.Structures and functionality presented as discrete components in theexemplary configurations may be implemented as a combined structure orcomponent. Various embodiments of the invention are contemplated toinclude circuits, systems of circuits, related methods, and tangiblecomputer-readable medium having encodings thereon (e.g., VHSIC HardwareDescription Language (VHDL), Verilog, GDSII data, Electronic DesignInterchange Format (EDIF), and/or Gerber file) of such circuits,systems, and methods, all as described herein, and as defined in theappended claims. In addition, the computer-readable media may storeinstructions as well as data that can be used to implement theinvention. The instructions/data may be related to hardware, software,firmware or combinations thereof.

The description of the invention set forth herein is illustrative, andis not intended to limit the scope of the invention as set forth in thefollowing claims. For example, while the invention has been described inembodiments in which a receiver receives OFDM communications over achannel including coaxial cable, one of skill in the art will appreciatethat the teachings herein can be utilized with devices consistent withOFDM communications over channels including other wireline or wirelesschannels. Variations and modifications of the embodiments disclosedherein, may be made based on the description set forth herein, withoutdeparting from the scope and spirit of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A method comprising: allocating a first number ofbits per symbol to a carrier tracking subcarrier of a plurality ofsubcarriers of an orthogonal frequency division multiplexing (OFDM)signal based on a first target performance margin; and allocatingnumbers of bits per symbol to other subcarriers of the plurality ofsubcarriers based on a second target performance margin.
 2. The method,as recited in claim 1, wherein the first target performance margin issubstantially greater than the second performance margin.
 3. The method,as recited in claim 1, further comprising: communicating the OFDM signalover a channel, the first number of data bits per symbol modulating thecarrier tracking subcarrier and the numbers of data bits per symbolmodulating the other subcarriers of the plurality of subcarriers.
 4. Themethod, as recited in claim 1, further comprising: communicating, over achannel, indications of the first number of data bits per symbolallocated to the carrier tracking subcarrier and the numbers of databits per symbol allocated to the other subcarriers.
 5. The method, asrecited in claim 1, further comprising: selecting the carrier trackingsubcarrier from the plurality of subcarriers of the OFDM signal based onperformance estimates for the plurality of subcarriers.
 6. The method,as recited in claim 5, wherein the selecting is based on a performanceestimate for the carrier tracking subcarrier as compared to performanceestimates corresponding to others of the plurality of subcarriers. 7.The method, as recited in claim 5, wherein the selecting comprises:determining as the carrier tracking subcarrier, a subcarrier of theplurality of subcarriers of the OFDM system having a maximum performanceestimate.
 8. The method, as recited in claim 5, further comprising:generating the performance estimates for the plurality of subcarriersbased on known received symbols.
 9. The method, as recited in claim 1,further comprising: selecting additional carrier tracking subcarriersfrom the plurality of subcarriers; and allocating numbers of data bitsto the additional carrier tracking subcarriers based on the first targetperformance margin.
 10. The method, as recited in claim 1, furthercomprising: generating a frequency offset indicator based on datareceived using the carrier tracking subcarrier.
 11. The method, asrecited in claim 10, further comprising: adjusting received data of anOFDM receiver based on the frequency offset indicator.
 12. The method,as recited in claim 1, wherein determining the first number of data bitscomprises determining a maximum number of data bits that provides thefirst target performance margin based on a performance estimate for thecarrier tracking subcarrier, and wherein determining the numbers of databits comprises determining maximum numbers of data bits that provide thesecond target performance margin based on corresponding performanceestimates for the other subcarriers.
 13. The method, as recited in claim1, wherein the first target performance margin is at least 3 dB greaterthan the second target performance margin.
 14. An apparatus comprising:a select module configured to select a carrier tracking subcarrier froma plurality of subcarriers of an orthogonal frequency divisionmultiplexing (OFDM) signal based on performance estimates for theplurality of subcarriers; and a bit allocation module configured toallocate a number of bits per symbol to the carrier tracking subcarrierfrequency based on a first target performance margin and configured toallocate numbers of bits per symbol to other subcarriers of theplurality of subcarriers based on a second target performance margin.15. The apparatus, as recited in claim 14, wherein the first targetperformance margin is substantially greater than the second targetperformance margin.
 16. The apparatus, as recited in claim 14, whereinthe first target performance margin is at least 3 dB greater than thesecond target performance margin.
 17. The apparatus, as recited in claim14, further comprising: a performance estimator configured to generate aplurality of performance estimates for the plurality of subcarriers. 18.The apparatus, as recited in claim 14, wherein the select modulecomprises: a maximum performance detector configured to identify as thecarrier tracking subcarrier, a subcarrier having a maximum performanceestimate of the plurality of performance estimates.
 19. The apparatus,as recited in claim 14, further comprising: a carrier tracking circuitmodule configured to generate a frequency offset indicator based on datarecovered from the carrier tracking subcarrier.
 20. A method comprising:communicating an orthogonal frequency division multiplexing (OFDM)signal over a channel, a carrier tracking subcarrier of a plurality ofsubcarriers of the OFDM signal being modulated by a first number of databits per symbol and other subcarriers of the plurality of subcarriersbeing modulated by numbers of data bits per symbol, the first numberbeing based on a first target performance margin and the numbers beingbased on a second target performance margin.
 21. The method, as recitedin claim 20, wherein the first target performance margin issubstantially greater than the second target performance margin.